Pumps - Specific Speed

Specific Speed is a number characterizing the type of impeller in pump in a unique and coherent manner. Specific speed is determined independent of the pump size and can be useful when comparing different pump designs.

• Specific Speed identifies the geometrically similarity of pumps

Specific speed can be expressed as

N_s = n q^1/2 / h^3/4                              (1)

where

N_s = specific speed

n = pump shaft rotational speed (rpm)

q = flow rate (m³/h, l/s, l/min, m³/min, U.S. gpm, British gpm) at Best Efficiency Point (BEP)

h = head rise (m, ft)

Online Specific Speed Pump Calculator

The calculator below can used to calculate the specific speed:

n - pump shaft rotational speed (rpm)

q -  flow rate (m³/h, l/s, l/min, m³/min, U.S. gpm, British gpm)

h = head rise (m, ft)

Note! When comparing pumps and their documentation - be aware of the units used.

Typical values for specific speed - N_s - for different designs in U.S. units (US gpm, ft)

• radial flow pump design - 500 < N_s < 4000 - typical for centrifugal impeller pumps with radial vanes - double and single suction. Francis vane impellers in the upper range
• mixed flow pump design - 2000 < N_s < 8000 - more typical for mixed impeller single suction pumps
• axial flow pump design - 7000 < N_s < 20000 - typical for propellers and axial fans

Convert Specific Speed between U.S. units (gpm) and Metric units (m³/h)

• N_{s(US gpm, ft)} = 0.861 N_{s(m³/h, m)}
• N_{s(British gpm, ft)} = 0.787 N_{s(m³/h, m)}

when

• 1 U.S. gpm = 0.2271 m³/h
• 1 British gpm = 0.2728  m³/h
• 1 ft = 0.3048 m
  

Example - Specific Speed for a Pump with different units

A pump has the capacity 1500 U.S. gal/min (1249 British gpm, 340 m³/h, 5667 liters/min) at 100 ft(30.5 m) of head and is rotating at 1760 rev/min.

US gpm, ft

N_{s(US gpm, ft)} = (1760 rev/min) (1500 gal/min)^1/2 / (100 ft)^3/4

    = 2156

British gpm, ft

N_{s(British gpm, ft)} = (1760 rev/min) (1249 gal/min)^1/2 / (100 ft)^3/4

    = 1967

m³/h, m

N_{s(m3/h, m)} = (1760 rev/min) (340 m³/h)^1/2 / (30.5 m)^3/4

    = 2500

liter/min, m

N_{s(liters/min, m)} = (1760 rev/min) (5667 liters/min)^1/2 / (30.5 m)^3/4

    = 10209

Specific Speed Hydro-power Turbines

Specific speed is also used when evaluating hydro-power turbines. Typical range of turbine specific speeds:

• Francis turbine : 70 - 500
• Kaplan or propeller turbine : 100 - 350
• Pelton, Turgo or Cross-flow turbines : 10 - 80